With the new Client Review site being released in Shotgun 6.3, its not possible for client users to approve versions right from within the CRS app.
We've heard from a few people it would be nice to be able to select which client users can approve versions, and which can simply comment and make notes. Perhaps a field on the client user entity dictating if they can approve versions?
To add your support to this feature, click on the 'Me Too!' button to vote it up, and feel free to add any additional thoughts or use cases in the comments.